PLD doesn't absorb magic either, nor is it a magic tank by default. AoE attacks were problematic for everyone not just tanks, which is why a lot of groups had people sub ninja for shadow and why a lot of HMN groups only took Ranged DPS . Third Eye however does absorb large hits, multi attack weapon skills, and auto attacks, which in turn allowed them solo tank slow hitting mobs, mobs that did multi hit weapon skills, and allowed them to get shadows back.
As for AOE and when Third eye are down. Thats what the DT sets are for since you can you know, change gear mid battle in that game.
Like the games current all around best tanking piece and rarest tanking piece?
http://www.ffxiah.com/item/26944/tartarus-platemail
The only job that has better DT gear is PLD and thats through Ageis and Burtang due to the fact that they let you break the Hard cap of -50% damage taken, which in case you didn't know no one can go above -50% damage taken naturally. These are two weapons that take most people months, if not years to obtain, which prior to them making Dynamis easy to get into by removing the time restraint and gil requirement, only a few people had their hands on these weapons and normally only one.
The only reason to take a PLD was for spike enmity and their ability to block so they could put their shadows back up.
PLD can naturally counter magic without - magic damage taken gear? Nope. RUN and PLD can counter AoE attacks naturally? Nope. So not sure why this really matters.
And if you're talking about SAM's ability to counter attack with Third eye up, of course they can't. But extra damage going out while their putting defenses back up is better than no damage at all. Thing about 11 is that you geared your Sub job to handle the fight you were going into. :0
IE: Mob does a lot of fast but weak attacks? No need for /ninja. Mob does a lot of single target Magic nukes and hits hard enough that your -50% magic damage taken gear set doesn't cut off enough damage? go /nin and now /run.
Kinda hard to with a game that has lost a good majority of it's player base. But from way back when. Sure.
http://www.ffxiah.com/forum/topic/15645/sammnk-tank/
You can search the rest of the old forums yourself since theres about....15 years worth of it.